(05-10) 20:52 PDT SAN FRANCISCO -- The family of slain BART passenger Oscar Grant and several of his fellow riders can go to trial in their claims that transit police arrested them illegally and used excessive force on the platform where Grant was shot to death, a federal judge ruled Tuesday.

The evidence is in dispute about whether Bay Area Rapid Transit officers had legal grounds for arresting the passengers after a reported fight on a train Jan. 1, 2009, and whether Officer Johannes Mehserle was justified in using any force against Grant, said U.S. District Judge Marilyn Hall Patel of San Francisco.

Mehserle, who testified he thought he was firing his Taser stun gun instead of his pistol, was convicted of involuntary manslaughter by a Los Angeles jury in July and sentenced to two years in prison. He is scheduled for release next month.

Mehserle said Grant was resisting arrest and appeared to be reaching into his waistband as he lay face down on the platform at Oakland's Fruitvale Station, with two officers kneeling on him.

But Patel said other evidence, including videos, suggested the 22-year-old Hayward man "was completely incapacitated and posed no threat to anyone." She said a jury should decide whether any force, including a stun gun, was justified.

She said a jury should also determine whether BART Officer Anthony Pirone had reason to believe any of the passengers had been violent or posed a threat when he ordered their arrests. BART later fired Pirone, who is appealing his dismissal.

The judge dismissed the passengers' claims that BART's failure to train and supervise its officers adequately contributed to their alleged misconduct. But the agency would have to pay damages if any of its officers was found liable.

BART agreed in January 2010 to pay $1.5 million in a settlement with Grant's 5-year-old daughter. The federal suit seeks additional damages.